1. Field of the Invention
This invention relates generally to pumps and more particularly to piston pumps for increasing fluid pressure.
2. Description of the Prior Art
Fluid intensifiers utilize the energy of a fluid at low pressure to pump a fluid at a higher pressure. Fluid pumps utilize the energy of a driving fluid to pump a driven fluid.
A piston type intensifier is described in U.S. Pat. No. 4,212,597 of Mallofre. This intensifier includes a main cylinder provided with a high pressure outlet, a spool cylinder provided with a low pressure inlet, a spool disposed within the spool cylinder, and a multihead piston disposed within the main cylinder. When a pressurized fluid source is applied to the low pressure inlet, the multihead piston is caused to reciprocate within the main cylinder assembly to develop a high fluid pressure at the pressure outlet.
A problem with piston type fluid intensifiers as described in the Mallofre patent is that they require a very stable continuous fluid force in order to operate. Piston type fluid intensifiers tend to become stuck in mid-cycle if the fluid source is interrupted because they depend on the momentum of the continuous operation to control the stroke direction of the pump pistons. If the pump pistons are stopped in mid-cycle by a loss of fluid source pressure, they could equally well move in either direction when fluid source pressure is reapplied. Rather than move in one direction or the other, the pump pistons often jam.
Some prior art piston type fluid intensifiers address this problem by providing biasing mechanisms to prevent the intensifier mechanism from being immobilized in mid-cycle. For example, Wrigley, in U.S. Pat. No. 2,826,149 provides a spring loaded overcenter mechanism for just such a purpose. Problems with biasing mechanisms is that they too can become stuck and that they add to the cost of the intensifier.
In U.S. Pat. No 2,818,022 of Kangas, the moving piston contacts a spring loaded switch at each end of the stroke which redirects the influx and efflux of fluid so that the piston reverses direction to maintain the pumping action and pressure intensification. Since the momentum of the pistons is not required to maintain pumping action at midstroke, the pumping action can be resumed in spite of temporary interruptions of fluid flow. This pump is intended for uses such as using a hydraulic power supply to pump fluid (oil) from deep underground where it is required to pump fluid from two different levels simultaneously. In this arrangement, the discharged fluid is two separate mixtures. Each mixture contains the hydraulic fluid and one of the fluids pumped from the ground.
U.S. Pat. No. 4,523,895 of Silva also eliminates the mid-cycle sticking problem associated with piston pumps by eliminating dependence on momentum of the piston in midstroke. This is accomplished by the piston face pushing a collar at each end of the stroke. The collar, concentric with the bore of the piston cylinder, moves first in one direction to cover a first set of orifices and uncover a second set of orifices so that the redirected flow of fluid reverses the main valve position reversing the motion of the piston; then the collar is moved in the reverse direction at the other end of the stroke to restore the original direction of fluid flow and motion of the piston.
In the construction of the pump described in the Silva patent, the collar positioned in the cylinder of the piston directs fluid flow to either end of a spool, slideably enclosed in a housing which is separate from the main piston cylinder. Grooves, circumferentially located on the spool, connect the fluid inlet to either end of the piston and the flow lines for fluid displaced from the low pressure end of the piston cylinder to the exhaust exit. The necessarily small cross sectional area of these flow channels of this construction imposes a constriction which limits the rate of flow through the pump. Furthermore, the additional flow lines required for the construction of the pump according to the Silva patent also imposes an additional expense.